Some LED printer heads employ an optical system containing an SLA (Selfoc Lens Array: registered trademark of Nippon Sheet Glass Co. Ltd.) which contains an array of gradient-index lenses for forming one continuous image in its entirety.
The LED constituting the LED printer head generates heat in light emission. To offset the adverse effect of the heat generated by the LED, Japanese Patent Application Laid-Open No. S62-249778 employs a finned member attached to the base plate carrying the LED array.
The above-mentioned SLA itself is constituted of a glass material. The inventors of the present invention considered use of a plastic material instead of the glass material for the lens array typified by the SLA for further decrease of the weight and reduction of the production cost.
The inventors of the present invention noticed that the lens array constituted of a plastic instead of the glass can be affected by the heat generated by an adjacent LED light-emitting element. The SLA (trade name of Nippon Sheet Glass Co. Ltd.) is a kind of a lens array, in which the rod lenses are formed from a transparent fiber useful as an optical fiber. In the transparent fibrous lens, the refractive index is changed from the surface toward the inside concentrally to obtain the lens effect. To decrease the interference between the adjacent rod lenses to prevent occurrence of stray light, the side wall of the rod is shielded from light by a resin or a like material. Further, a technique is disclosed of laminating a microlens array on a glass plate or a plastic plate by lithography or a like process to obtain an erect equal-sized lens effect. This method is advantageous in that deviation of optical axes among the rods is practically decreased in bundling of the rod lenses in an array. The present invention is applicable in the above-mentioned two types of lens arrays.